1. Field of the Invention
This invention relates to bulkhead structure for multi-stage rocket motors and especially to a reverse bulged forward acting scored rupture disc bulkhead capable of withstanding extremely high pressures thereagainst in one direction while opening fully under a relatively low pressure in the opposite direction to allow free flow of gaseous propellant products of combustion therethrough.
2. Description of the Prior Art
Multi-stage rocket motors have found increasing usefulness in recent years because of the increased sophistication and miniaturization of missiles capable of being launched from mobile air, ground or water-based platforms. These missiles usually embody at least two propellant stages. An ignitor ignites the propellant stage in closest proximity to the discharge port of the missile casing, and additional stages are then ignited in successive order depending upon the nature of the launch and delivery trajectory established for a particular weapon.
In order to preserve the integrity of the desired missile track, especially over long distances, the propellant stages must be ignited at times which are very critical one with respect to another. If a second propellant stage is prematurely ignited before burning of the first stage has taken place to a predetermined extent, the accuracy of the missile can be compromised. Furthermore, if ignition of successive stages is delayed and does not occur at a precise time relative to preceding propellant stage ignition, the track of the missile can either be undesirably altered, or a programmed or command change will not occur as required for target impact.
A bulkhead rupture disc assembly used in a multi-stage rocket motor must not only reliably prevent backflow of gaseous propellant reaction products from the stage undergoing burning, but also must substantially instantaneously open under a much lower pressure thereagainst in the opposite direction so that there is efficient discharge of propellant combustion products. Furthermore, the bulkhead must prevent such backflow of combustion products from a preceding propellant stage while at the same time freely opening in the opposite direction upon ignition of the next succeeding stage without permitting the gaseous products from being discharged unevenly across the discharge port of the missile which will alter its directional track. Similarly, full opening of the disc must be assured to avoid obstructions in the gaseous combustion products flow path which under certain conditions might result in an explosion of the missile.
An example of the utilization of a rupture disc in a multi-stage rocket motor for missiles is illustrated and described in U.S. Pat. No. 4,738,099 of Apr. 19, 1988 which issued to the assignee hereof. In the '099 patent, a rupture disc having a series of radially extending lines of weakness projecting from the center of the disc is described for a missile having multiple propellant stages. The rupture disc has a central aperture for accommodating an axially positioned ignitor assembly for effecting ignition of the succeeding solid propellant stage.
The bulkhead rupture disc assembly of the '099 patent incorporates a dome shaped, centrally apertured metal disc supported by a complementary apertured dome support of a synthetic resin such as a phenolic composition. Although the assembly described and illustrated in that patent has utility in a number of rocket motor applications for missiles and the like, it cannot be used in small diameter rocket motors which are unable to accommodate a centrally located ignitor that is positioned within an opening in the center of the bulkhead. A bulged rupture disc unit similar in certain respects to that of the '099 patent was tested in the early 1970's by the assignee hereof, but the unit cannot be used in the present multi-stage rocket motor application because it has limited back pressure capabilities. That assembly has a large scored concavo-convex metal disc (about 21/2 inches in diameter) wherein the convex face is supported against a domed metal plate of similar thickness and having a series of small perforations defining a cross-shaped pattern. The concave surface is cross scored before bulging of the disc across the entire diameter thereof which bridges the relief passage. The opening pressure is about 350-750 psi; the withstand backpressure is limited to 2000 psi or less.
Another known rupture disc construction is illustrated and described in U.S. Pat. No. 4,505,180 of Mar. 19, 1985 which also issued to the assignee hereof. In the construction of the '180 patent, a rupture disc is provided with a metallic, frangible disc having a low pressure face and an opposed, scored high pressure face, along with an apertured backing member positioned adjacent the low pressure face. The backing member includes a plurality of webs aligned with the disc scoring which engage the low pressure face of the disc and prevent disc rupture under the influence of high burst pressures directed against the remote high pressure face, while permitting disc rupture when the relatively low burst pressures are developed adjacent the low pressure face.
Although the device disclosed in the '180 patent finds utility in a multi-stage rocket motor for missles, a number of limitations exist with the known construction. For example, it would be desirable able to provide a rupture disc assembly that has a reduced weight relative to the known construction and that further expedites a free and full opening of the disc and backing member in order to provide a clear flow path for gaseous combustion products discharged past the assembly.